Microelectronic circuits, in the form of integrated circuits, are constructed in a multitude of process steps. Among the most critical steps is the etching process. Etching is the process by which circuit patterns are permanently transferred onto the surface layer of a semiconductor wafer. The etching process removes the top layer(s) of the wafer surface through openings in a surface pattern. The pattern of these openings are known as the mask. The primary goal of etching is to transfer an exact mask pattern into the wafer surface. Other objects of etching are pattern uniformity, edge profile control, selectivity, cleanliness, and low cost.
In general, etching falls into two main categories; wet and dry. Historically, etching has been accomplished by immersion in, or spraying on of wet etchants. (In general, etchants are acids which are applied to the surface of the wafer for a specified time, selectively dissolving the wafer surface in unmasked areas, allowing a specific pattern to be etched into the wafer surface.) Wet etch techniques are effective when the feature size of the microelectronic device is larger than 3 .mu.m in size. However, in the ongoing pursuit of smaller feature sizes, dry etching was developed. Dry etch techniques include: plasma etching, ion beam etching, and reactive ion etching (RIE). All of these techniques, dry and wet, have significant limitations. Wet etchants suffer from isotropic effects, which result in unintentional non-uniform sloping of the side walls in the etched areas. The longer the etch time, the more magnified the effect. Dry etch techniques suffer from a variety of different limitations including radiation damage, photoresist hardening (making the resist hard to remove after the etching is finished), difficulty in maintaining etch profile, unpredictability of results over long etch times (especially in deep trenches with high aspect ratios), dilution effects which reduce the effectiveness of the process, and poor selectivity. All of these effects are well known in the art.